I mean sure I can see Isaac had his reasons, and perhaps they weren't all out of bad faith. But here's the thing, Rubia refused. She clearly said she was fine with it and will withstand torture for our skele's sake. Isaac did not respect her wishes. I believe that invalidates the euthanasia argument, and changes his actions to simple murder.
I do still hope they make up in the end though. Maybe, in next life?